backup anchor while on mooring

Does anyone else use a backup anchor while on a mooring with strong winds with the unlikely change of wind or water current

This is not about jumping in and inspecting the mooring and tackle( I do ) or scope required for a properly set anchor but just dropping the anchor in shallow waters with a little bit of slack at a mooring with during strong winds

Generally this is not done.
If you get blown around the mooring the anchor line and mooring line will get tangled together.

Bad idea, if the winds are strong you are required to be off the mooting anyway. Two anchors is a better solution.
George

Not a useful practice -- 'with a little slack' means it wouldn't hold or help in high winds, and will be off the bottom drifting into deeper water.

You may even damage the mooring equipment if it tangles up and you use a windlass to try and pull it back aboard.

Do the charter companies equip the bareboats with two anchors?

Moorings does...although if I ever see rough weather ahead I am taking George...:)

More often done with a line to shore to keep the boat from swinging but rarely seen in BVI, more St Lucia area. This would not help if mooring failed.

At what strength winds are you required to be off the moorings?

40 knots